Microstucture And Machanical Property Of Ultra-high Strength Mg-Y-Ni Alloys

In this thesis prepared a series of Mg-Y-Ni alloy by gravity casting method and discussed the influence of Ni element and element for Mg-Y-Ni alloy by microstructure and mechanical properties. And use Pandat 8 for alloy solidification process simulation to explore the regular of Y element and Ni element for as-cast Mg-Y-Ni alloy microstructure and mechanical properties.In order to make the second phase back to the matrix as far as possible we take the method of homogenization treatment.And then,we take the same extrusion parameters of hot extrusion deformation to research the relationship between the microstructure and properties of alloy.Though changing the content of Ni element,we prepared four kinds of Mg-Y-Ni alloys,such as WN72,WN75,WN77 and WN79.And found there are α-Mg、Mg2Ni and LPSO phase.The structure of α-Mg is dendrite shape and LPSO phase present continuous mesh structure.Mg2 Ni phase distribute between α-Mg and LPSO interface.Among the four kinds of alloys,the Mg2 Ni phase exceed LPSO phase in WN79 and makes the as-cast mechanical properties at room temperature very poor.After homogenization treatment four kinds of alloys all use the same hot extrusion process(extrusion temperature: 400 ℃; extrusion ratio: 12; extrusion speed:10 mm/min) to explore the influence of Ni element for the microstructure and mechanical properties of extrusion state.We found the proportion of dynamic recrystallization grow from WN72 to WN79 and the matrix grow up abnormally in WN79.LPSO phase elongated along the extrusion direction.The mechanical properties become poor owing to excessive Mg2 Ni making the LPSO phase broken in alloy. And in WN75, quasi-continuous mesh LPSO phase make the highest mechanical properties at room temperature among the four alloys.After homogenization treatment we research the influence of temperature of extrusion for the microstructure and mechanical property. We choose the extrusion temperature of 350 ℃ and 400 ℃ respectively.Though XRD texture testing found macro-texture intensity reduced when the temperature of extrusion decrease from400℃ to 350℃, this is mainly due to dynamic recrystallization completely occurred in 400℃. Though the tensile properties test,the yield strength and tensile strength of300℃ is higher than 400 ℃.We prepared two kinds of alloys though changing Y element content.There are four kinds of phases in the alloys such as α-Mg、Mg2Ni 、LPSO phase and another phase which composition is similar to LPSO phase.In As-cast WN155 has the excellent mechanical properties.And after homogenization treatment we take the same extrusion parameters(extrusion temperature: 450 ℃; extrusion ratio: 12 extrusiospeed: 10 mm/min) to explore the influence of Y element content for the microstructure and mechanical properties of extrusion state.We found low proportion among WN155 matrix dynamic recrystallization though OM,SEM and XRD macro-texture test.Most elongated matrix arrange along the extrusion direction, so the texture strength is higher. In the room temperature tensile mechanical properties tests found the yield strength and tensile strength of WN155 are much higher than WN105,but the elongation rate is much lower. This shows texture reinforcement is not the main reason for the high strength, but the real reason is the LPSO phase in the extrusion process of This shows texture reinforcement is not the main reason for the high strength alloy, and the real reason is the kink belt reinforcement of the LPSO phase in the extrusion process.